Efforts to control the transmission of infrared and thermal radiation through windows in structures such as homes, office buildings, aircraft, and automobiles is of utmost interest in areas of energy savings and temperature modulation. Current technologies for such windows are generally based on low-e glass and materials that allow modulation of selected radiation, which include electrochromic, thermochromic, and photochromic materials. The most adaptable for energy savings applications, in an actively controlled manner, is based on electrochromics. These windows are based on two transparent electrodes onto which electrochromic active materials are coated, with an electrolyte separating the coated electrochromic materials. The electrochromic materials that have been explored include metal oxides, small organic molecules, and conjugated polymers. These windows do not allow tunable control of IR transmission, independent of visible region modulation.
The operating principle for electrochromic cells is similar to that of a battery, where one electrochrome-coated electrode acts as an electrode, for example, an anode, and the other acts as a counter electrode, for example, a cathode. Most high-contrast cells comprise an actively coloring material at the anode and a charge balancing material at the cathode, where the charge balancing material displays complementary coloring properties to the coloring material, or exhibits little or no color change. Typically, when no voltage is applied to the electrochromic cell, absorption of radiation occurs in the visible region and the cell is in a colored state. When a positive voltage is applied to the cell, a bleached state with high transmission in the visible region is achieved.
Operation of the cell requires complementary materials, where one is oxidized and the other is reduced. State of the art electrochromic windows have a visible region optical modulation that is rather high, while long wavelength (NIR and IR) optical changes are minimal, where one of the materials is always highly IR absorptive during device operation. As such, there is a large amount of visible light modulation, with the window switching between tinted and non-tinted states, providing limited privacy and glare reduction, but allowing little ability to actively modulate thermal radiation. FIG. 1 illustrates this modulation for a typical electrochromic device based on a conjugated polymer electrochrome at one electrode (ECP-black) and a minimally coloring material (MCCP) at the counter electrode, where the device has a high contrast in the visible region (40% ΔT at 555 nm), but little contrast in the NIR (18% ΔT at 1200 nm and 4.2% at 2000 nm). This pairing of materials results in maximal transmittance of NIR radiation when the device is least visibly light transmissive, which is contrary to what one would desire for a window to attenuate heat from sunlight on a warm day, yet allowing transmission of NIR radiation on cold days and thermal control in the evening when visible light modulation is not as important.
Hence there remains a need for a device that can modulate IR radiation for heat control in structures and vehicles including buildings, automobiles, and airplanes. Typically, during a hot day, it is desirable to keep the indoor cooling costs to a minimum. Therefore, a window that has high IR absorption is desired. In contrast, on a cold day, it is advantageous to have a window with low IR absorption to keep indoor heating costs to a minimum. Therefore, an electrochromic cell that allows control over IR transmission with little to no visible light variation is desirable.